Improve our default new Merge Request template

What does this Merge Request do?

This MR firstly updates the current default new MR template at c3049363 and then updates to incorporate improvements discussed at #1189 (closed).

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #1189 (closed)